Conventional flame combustion of solid, liquid, and gaseous fuels has been utilized for hundreds of years to provide heat for use directly, or indirectly to provide energy to drive other processes. Applications which have utilized this conventional flame combustion technology include furnaces, boilers, gas turbine engines, etc.
As concern for the environment have increased, and as the scientific understanding of combustion processes has improved, it is now an indisputable fact that emissions from conventional flame combustion of hydrocarbon fuels, is a major source of airborne pollutants. The current rate of production of these various combustion related airborne pollutants is creating an adverse impact on the environment, which quite potentially endangers the existence and well being of many plant and animal species on earth, including the human race. The recognition of this problem, has led to increased exploration in combustion technologies which can reduce the amount of airborne pollutants, and has resulted in increased governmental regulations on the various pollutants in an effort to minimize or eliminate their impact on the environment.
Certain pollutants such as sulfur-dioxide (SO.sub.2), are fuel based pollutants. The removal of these fuel-based pollutants may be accomplished through a treatment process applied directly to the fuel itself, or by post- combustion exhaust treatment to remove the pollutant directly therefrom in its post-combustion state.
Other pollutants, such as carbon monoxide (CO) and unburned hydrocarbons (ULIC's), are generated as a result of the incomplete oxidation of the fuel used in the conventional combustion process. Contaminants such as this may be removed through post-combustion oxidation, or by improving the combustion process itself, to more completely combust the fuel. One way to improve the complete combustion of the fuel is to increase the temperature of the air and/or fuel components. Elevation of the combustion temperatures ensures that the temperature achieved during combustion are high enough to promote completion of the combustion reaction. However, as the amount of carbon monoxide is reduced by these elevated temperatures, emissions of NO.sub.x increase exponentially (see FIG. 1). NO.sub.x is a compound of Nitrogen and Oxygen, which contains both NO as well as minor amounts of NO.sub.2. NO.sub.x is a pollutant generated in a complex reaction which occurs when atmospheric nitrogen and oxygen chemically recombine at elevated temperatures. The amount of NO.sub.x produced increases as temperature and pressure increase. Thus NO.sub.x is a by-product pollutant which is generated as a result of combustion processes, particularly internal combustion used in reciprocating and combustion gas turbine machinery.
As with other pollutants, removal of NO.sub.x may be accomplished through control of the combustion process or by removal of this pollutant from the exhaust. One way to remove NO.sub.x from the exhaust gas, (as applied in automotive applications) is to chemically react the NO.sub.x with CO to produce nitrogen and carbon dioxide (N.sub.2 and CO.sub.2). However, in some high temperature combustion applications, such as in a gas turbine engine, the amount of available CO is insufficient to remove the large amounts of NO, produced via the high combustion temperatures.
A conventional flame combustion process, such as that used in a gas turbine engine (see FIG. 2), operates on a cycle whereby intake air is pressurized by a rotating compressor 10. This pressurized air is passed through a chamber or "combustor" 12 wherein fuel is mixed with the air and burned. The high temperature combustion gases are allowed to expand across a rotating turbine 14 which results in a torque imparted to the turbine shaft 16. Typically, the turbine and compressor are connected to a common shaft, such that the torque created by the turbine serves to drive the compressor, thus completing the cycle. In addition, since the torque produced by the turbine greatly exceeds the "parasitic load" of the compressor, the same shaft may be coupled to an external load. It is this external shaft work which makes the gas turbine useful as a source of mechanical energy. Gas turbines are a common engine design used to power turbo-prop aircraft, electrical generators, pumps, compressors, and other devices requiring rotational shaft power.
In a typical gas turbine engine, the combustion chamber, fuel delivery system, and control system are designed to ensure that the correct proportions of fuel and air are injected and mixed within one or more "combustors." A combustor is typically a metal container, or compartment, where the fuel and air are mixed and burned. Within each combustor, there is typically a set of localized zones where the peak combustion temperatures are achieved. These peak temperatures commonly reach temperatures in the range of 3300 degrees Fahrenheit. These high temperatures also become the source of NO.sub.x emissions. Typically, to prevent thermal distress or damage to these metallic combustion chambers, a significant amount of the compressor air passes around the outside of the combustors to cool the combustors. The air which then drives the turbine is a combined mix of the hot combustion gasses and this cooling air. The resulting, hot gas yield which is admitted to the inlet of the turbine is delivered at a temperature in the range of 2400 degrees F at full load for a typical industrial gas turbine. Unfortunately, virtually all of the NO.sub.x produced in the peak temperature zones within the combustor is exhausted into the atmosphere.
In an effort to reduce the amount of pollutants generated and released by the combustion of fuel, significant effort has been expended to develop a flameless combustion process useable in furnaces, boilers, gas turbines, etc. One such flameless combustion process, developed by Catalytica Combustion Systems Inc. of Mountain View California, uses a catalyst module design which employs a honeycomb-like construction. Unique chemicals, imparted onto the interior surfaces of the honeycomb structure, serve to augment the chemical reaction of the fuel and air. This "distributed combustion" allows complete combustion of the constituents to occur at relatively low temperatures, and with comparatively low concentrations of fuel. Due to it's construction, the heat produced by the catalytic module occurs over a large zone and occurs very uniformly, eliminating the "hot zones" typical in traditional gas turbine combustors. As noted earlier, these "hot zones" are where NO.sub.x emissions originate within traditional combustion systems.
Catalytica Combustion Systems Inc. of Mountain View California has developed and patented pioneering work in the field of catalytic combustion as evidenced by the following U.S. Pat. Nos. 5,511,972; 5,518,697; 5,512,250; 5,425,632; 5,461,864; 5,405,260; 5,326,253; 5,281,128; 5,259,754; 5,258,349; 5,248,489; 5,248,251; 5,232,357; and 5,183,401. These patents describe and claim various constructions and methods to control the catalytic combustion process. These patents also describe and claim increases in combustion efficiency and reductions in the amount of undesired combustion byproducts or "pollutants" resulting therefrom. The descriptions and teachings of the catalytic combustion processes and structures of these references are hereby incorporated by reference.
While the aforementioned patents describe pioneering work in the development of a working catalytic combustor, their direct application to a dynamic system is somewhat more limited. Specifically, each of the aforementioned patents describe a control process in the context of steady state operation, at various fixed, steady state conditions. These step wise, steady state conditions are not representative of the continuous, dynamic nature of a combustion system when such combustion systems are applied on a gas turbine engine, furnace, boiler, or other combustion dependent process or plant. The nature of the application of combustion systems typically necessitate a means to smoothly transition throughout a range of operating conditions. Furthermore, many combustion systems require management of various time response characteristics for various performance and safety related reasons. Within the context of the larger, more complex, combustion dependent process, the catalytic combustion system requires an appropriately designed, dynamic control system to manage the multiple complex dependencies required to achieve proper operation of the catalytic combustion process. In addition, the control system must also monitor, and manage these combustion processes over a continuous range of static conditions, while providing appropriate dynamic performance as dictated by the larger system; i.e. the gas turbine engine, furnace, boiler, etc.
In general, a combustion system is used as a subsystem of a larger, more complex process (referred to subsequently as the "Plant") The Plant could be a gas turbine powered generating package, with its compressor, turbine, and generation sub-systems, a paper manufacturing plant requiring a steam generator for pulp processing and drying, a material processing plant which produces refined metals, crystals, or ceramics from raw materials, or any number of other processes or applications which rely on the heat energy released by the combustion process. Each of these larger processes requires that the combustion process be regulated appropriately to satisfy the final objective of the plant or process. Generally, some form of control system has been developed to regulate these traditional (non-catalytic) combustion processes. These Plant control systems have been designed, developed, and refined to manage the specific needs of the overall processes, including the requirements to vary the combustion process over vastly varying, dynamic, real time operating conditions. In the majority of combustion applications, dynamic requirements must be maintained to ensure that the process requirements are always in control. The Plant control system generally ensures that the thermal limits, pressures, and rates of change are properly maintained within the design limits of the process. Due to the tremendous energy released by an industrial scale combustion process, failure to maintain control of these combustion process can result in situations which may result in loss of life, significant financial loss, and/or significant environmental damage. More modern Plant control systems must also monitor and regulate the amount of pollutant gasses admitted to the atmosphere. Catalytic combustion systems also require an appropriate control system, to ensure that the requirements of the overall process are maintained, but additionally, require a control system to ensure that the process conditions required for proper operation of the catalytic combustor are maintained.
One system which could benefit from the utilization of catalytic combustion is a gas turbine engine. These machines are commonly used to power large, commercial, electrical generators. In this exemplary environment, the requirements of the overall system cover a wide range of operating and dynamic requirements. Operating conditions may change almost instantaneously, as the system of mechanical components, some rotating at high speed and generating large amounts of heat, pressure, and horsepower, must suddenly transition to a no load condition, at the sudden opening of an electrical circuit breaker. In this example, the control system must be able to reduce the output of the combustion process in a fraction of a second to ensure that the machine does not overspeed beyond it's design limits. In addition, operation of the combustion system must be regulated to ensure that the combustion components are maintained within safe limits. It is the function of the control system(s) to continuously regulate the overall Plant as well as the combustion subsystems.